1. Field of the Invention
The present invention relates to the technical field of switching power supply, and more particularly, to a system for transferring an alternating voltage into a direct voltage.
2. Description of the Related Art
Since the electrical technologies have developed rapidly, the application of the wireless network might be appeared in various hand-held devices. In the hand-held device, like a mobile phone, a PDA or a digital camera, a power supply is prerequisite standard equipment. The hand-held devices might be charged or used with a power supply. In the prior art, a linear power supply is used to charge the various hand-held devices typically. However, the transform efficiency of the linear power supply is very low and the swing of the input voltage is small. In generally, the input voltage swing is about ±10% of the input voltage.
To overcome the problem of the linear power supply, a high-frequency switching power supply can raise the transform efficiency for transferring the alternating voltage into the direct voltage. FIG. 1 is a circuit diagram of a typical switching power supply. As shown in FIG. 1, when a power MOSFET 110 is in turn-on state, the input voltage Vi would be supplied to a primary winding 121 of a transformer 120. The current of the primary winding 121 would increase gradually and store the energy in the primary winding 121, and then induct the energy to a secondary winding 122 of the transformer 120. When the power MOSFET 110 is in cutoff state, the current ID which goes through the power MOSFET 110 will be overlapped with the voltage VD of the power MOSFET 110 in time domain and a switching loss would be produced. FIG. 2 is schematic view of a switching loss of the switching power supply. As shown in FIG. 2, the current ID waveform of the power MOSFET 110 is overlapped with the voltage VD waveform. The overlapped area is the switching loss. The switching loss not only wastes energy, but also decreases the transform efficiency.
Therefore, it is desirable to provide a system for transferring an alternating voltage into a direct voltage to mitigate and/or obviate the aforementioned problems.